The present invention relates to a liquid crystal driving method and a liquid crystal driving circuit, and more specifically to a liquid crystal driving method and a liquid crystal driving circuit, which can control unevenness in color in a liquid crystal panel, attributable to a voltage shift occurring in the case of carrying out an AC driving on the basis of a potential on an opposing electrode in the liquid crystal panel.
In general, a liquid crystal panel is written with several tens frames (several tens screen images) per second, and an output signal of a liquid crystal drive circuit carries out an AC drive on the basis of a potential on an opposing electrode in the liquid crystal panel, in units of scan line or in units of frame. Namely, if an DC voltage continues to be applied to the liquid crystal, ions are accumulated in one electrode, with the result that the liquid crystal becomes immediately deteriorated. In order to avoid this deterioration, the AC drive is carried out by inverting, in units of one frame or a few frames, the positive/negative polarity of the liquid crystal drive circuit output signal, which is a video signal voltage to be applied to the liquid crystal.
FIG. 7 shows an example of the liquid crystal drive circuit for carrying out the AC drive on the basis of the potential on the opposing electrode in the liquid crystal panel in the above mentioned manner. This liquid crystal drive circuit is a technology disclosed in Japanese Patent Application Pre-examination Publication No. JP-A-09-218671, and is a switched capacitor type D/A converter having a sample period and a hold period. This D/A converter is mainly constituted of a differential operational amplifier 304 connected to an output terminal 303 and having a non-inverted input terminal connected to a first reference voltage input terminal 300. An inverted input terminal of the differential operational amplifier 304 is connected to a first capacitor group 305 including a plurality of capacitors which are constituted of unitary capacitors as a basic element. A second capacitor or a second capacitor group (which will be expediently generically called the second capacitor group in this description) 306 is connected between the non-inverted input terminal and the output terminal 303 of the differential operational amplifier 304. In addition, the following switch group is constituted for on-off switching between the differential operational amplifier 304, the first capacitor group 305 and the second capacitor group 306.
Namely, a first switch group 307 is provided in which one end of each switch is connected to one end of a corresponding capacitor in the first capacitor group 305 and the other end of each switch is connected in common to a second reference voltage input terminal 301. A second switch group 308 is provided in which one end of each switch is connected to one end of a corresponding capacitor in the first capacitor group 305 and the other end of each switch is connected in common to a connection node between third and fourth switches 309 and 310 explained hereinafter. There are provided the third switch 309 having one end connected to the other end of the second switch group 308 and the other end connected to the first reference voltage input terminal 300 and the non-inverted input terminal of the differential operational amplifier 304, the fourth switch 301 having one end connected to the other end of the second switch group 308 and the one end of the third switch 309 and the other end connected to a third reference voltage input terminal 302, and a fifth switch 311 connected in parallel to the second capacitor group 306.
In this liquid crystal drive circuit, two values are selected from gamma-compensated analog gradation voltages of for example 8 to 10 gradation levels, which are supplied from an external circuit of the drive circuit, and the two selected values of the analog gradation voltages are supplied to the second and third reference voltage input terminals 301 and 302, respectively, and on the other hand, the first to fifth switch groups and switches 307 to 311 are selected turned on, so that an analog gradation voltage is further divided with the result that one level of multi-gradated gradation data is outputted from the output terminal 303 as an analog image data. In addition, the polarity of the voltages applied to the second and third reference voltage input terminals 301 and 302 is inverted in order to carry out the AC drive. Incidentally, the inversion of the polarity of the reference voltage generates a large load when the liquid crystal drive circuit is operated. Therefore, the above referred Japanese publication discloses that a control circuit is provided for selectively operating each of the above mentioned switches. This control circuit receives a digital image data, a sample/hold input clock and a frame input clock, and inverts the polarity of the voltage outputted from the output terminal, on the basis of the voltage on the first reference voltage input terminal 300, in accordance with the image data and the clocks. However, the detail will be omitted.
However, in the liquid crystal drive circuit shown in FIG. 7, since the output voltage is determined by a ratio between the first capacitor group 305 and the second capacitor group 306, if the value of this ratio varies, the output voltage is deviated from a set value. For example, in the process of a fabrication of the liquid crystal drive circuit, when a reticle is prepared or when a capacitor is actually shot onto a wafer, if a difference occurs in capacitance between the first capacitor group 305 and the second capacitor group 306, by changing from one circuit to another, from one chip to another, from one wafer to another, and from one lot to another, an error occurs in the output voltage as mentioned above, with the result that a display unevenness attributable to the output voltage difference occurs in an image displayed in the liquid crystal.
This output voltage difference can be specifically expressed by the following mathematical equations. Here, in order to simplify the calculation, it is assumed that the circuit shown in FIG. 7 is a 2-bit switched capacitor type D/A converter. When the value of the first capacitor group 305 is deviated from the value of the second capacitor group 306 by a capacitance value xcex94xcex1 in a capacitance increasing direction, the voltage value of a positive side is expressed by the equation (1):
Vout(positive, xcex1)={Vref(4+xcex94xcex1)}/(4+xcex94xcex1) xe2x88x924V2/(4+xcex94xcex1)xe2x88x92{"khgr"(V1+V2)}/(4+xcex94xcex1)xe2x80x83xe2x80x83(1)
In a similar condition, the voltage value of a negative side is expressed by the equation (2):
Vout(negative, xcex1)=xcex94xcex1/(4+xcex94xcex1) +4V2/(4+xcex94xcex1)+{"khgr"(V1+V2)}/(4+xcex94xcex1)xe2x80x83xe2x80x83(2)
where Vref is a first reference voltage supplied to the first reference voltage input terminal 300, V1 is a second reference voltage supplied to the second reference voltage input terminal 301, and V2 is a third reference voltage supplied to the third reference voltage input terminal 302.
In the case of driving the liquid crystal panel, the AC drive is carried out by alternately outputting the voltage expressed by the equation (1) and the voltage expressed by the equation (2). However, if the capacitance value difference expressed by xcex94xcex1 occurs in each of the above equations, the amplitude of the voltage on the basis of the potential of the opposing electrode in the liquid crystal panel increases as shown in FIG. 8, or alternatively decreases, so that the output signal having an error is outputted. Therefore, an actually displayed color is expressed as an effective value=[(1)xe2x88x92(2)]/2. This effective value is expressed by the equation (3). Incidentally, the equation (3) is expressed in the form of A(xcex94xcex1) which is a function of xcex94xcex1.
Vout(xcex1)=4Vref/(4+xcex94xcex1)xe2x88x924V2/(4+xcex94xcex1)xe2x88x92{"khgr"(V1+V2) }/(4+xcex94xcex1)xe2x80x83xe2x80x83(3)
Next, when the value of the second capacitor group 306 is deviated from the value of the first capacitor group 305 by a capacitance value xcex94xcex2 in a capacitance increasing direction, the voltage value of a positive side and the voltage value of a negative side are expressed by the equations (4) and (5), respectively:
Vout(positive, xcex2)={Vref(8+xcex94xcex2)}/4xe2x88x92V2xe2x88x92{"khgr"(V1xe2x88x92V2)}/4xe2x88x92(V1xc2x7xcex94xcex2)/4xe2x80x83xe2x80x83(4)
Vout(negative, xcex2)=xe2x88x92{Vrefxc2x7xcex94xcex2)}/4+V2+{"khgr"(V1xe2x88x92V2)}/4xe2x88x92(V1xc2x7xcex94xcex2)/4xe2x80x83xe2x80x83(5)
Thus, an actually displayed color is expressed as an effective value=[(4)xe2x88x92(5)]/2. This effective value is expressed by the equation (6). Incidentally, the equation (6) is expressed in the form of B(xcex94xcex2) which is a function of xcex94xcex2.
Vout(xcex2)={Vref(4+xcex94xcex2)}/4xe2x88x92V2xe2x88x92{"khgr"(V1xe2x88x92V2)}/4xe2x88x92(V1xc2x7xcex94xcex2)/4/xe2x80x83xe2x80x83(6)
Accordingly, a difference in the output between the equation (3) and the equation (6) is observed as a color unevenness between circuit outputs, between chips, between wafers, and between lots. If the degree of this color unevenness is expressed by a output voltage difference xcex94V, the following equation (7) is obtained:
xcex94V=A(xcex94xcex1)xe2x88x92B(xcex94xcex2)xe2x80x83xe2x80x83(7)
Accordingly, it is an object of the present invention to provide a liquid crystal driving method and a liquid crystal driving circuit, which can cancel the above mentioned color unevenness attributable to the deviation of the capacitance in the capacitor groups.
A liquid crystal drive method in accordance with the present invention is characterized in that in a liquid crystal drive method for carrying out a gradation display by AC-driving a liquid crystal panel by use of a liquid crystal drive circuit constituted of a switched capacitor type D/A converter, the liquid crystal panel is driven by alternately changing, at every predetermined periods, the polarity of an output error appearing in the liquid crystal drive circuit. Here, the polarity of the output error is alternately changed in units of xe2x80x9cnxe2x80x9d frames (where xe2x80x9cnxe2x80x9d is integer not less than 1).
In addition, a liquid crystal drive circuit in accordance with the present invention is constituted of a switched capacitor type D/A converter having a sample period and a hold period, for AC-driving a liquid crystal panel, the liquid crystal drive circuit comprising a differential operational amplifier, a first reference voltage connected to one input terminal of the differential operational amplifier, a first capacitor group connected to the other input terminal of the differential operational amplifier, for dividing second and third reference voltages, a second capacitor group connected between an output terminal and the other input terminal of the differential operational amplifier, and switch means for changing a connection condition of the first capacitor group and the second capacitor group to the differential operational amplifier, the switch means being on-off controlled at every predetermined periods for changing the connection condition.
As shown in a conception diagram of FIG. 1, the liquid crystal drive circuit in accordance with the present invention is constituted of a switched capacitor type D/A converter having a sample period and a hold period, which comprises a differential operational amplifier 104, a first reference voltage input terminal 100 connected to a non-inverted input terminal of the differential operational amplifier 104, a first capacitor group 105 connected to an inverted input terminal of the differential operational amplifier 104 for dividing a reference voltage supplied from second and third reference voltage input terminals 101 and 102, a second capacitor group 106 connected between an output terminal 103 and the inverted input terminal of the differential operational amplifier 104, and a switching means 107 for changing a connection condition of the first capacitor group 105 and the second capacitor group 106 to the differential operational amplifier 104, so that by on-off controlling the switching means 107 at every predetermined periods, the connection condition is changed. Therefore, when there is an error or a variation in the capacitance value between the first capacitor group 105 and the second capacitor group 106 in the liquid crystal drive circuit so that an output error occurs which causes a color unevenness, the connection condition of the first capacitor group 105 and the second capacitor group 106 is exchanged by the switching means 107 at every xe2x80x9cnxe2x80x9d frames for example, so that the polarity of the appearing output error is inverted at every xe2x80x9cnxe2x80x9d frames, with the result that the appearing output error is canceled at every xe2x80x9c2nxe2x80x9d frames and therefore the color unevenness is canceled.